Albert Barnes on Philemon 1:11

Which in time past was to thee unprofitable. Either because he was indolent; because he had wronged him, (comp. Phm 1:18;) or because he had run away from him. It is possible that there may be an allusion here to the meaning of the name Onesimus, which denotes profitable, (from ονινημι, fut. ονησω, to be useful, to be profitable, to help;) and that Paul means to say that he had hitherto not well answered to the meaning of his own name, but that now he would be found to do so.

But now profitable to thee. The Greek here is ευχρηστον, euchreston, but the meaning is about the same as that of the word Onesimus. It denotes very useful. In 2Ti 2:21, it is rendered, meet for use; in 2Ti 4:11, and here, profitable. It does not elsewhere occur in the New Testament.

And to me. Paul had doubtless found him useful to him as a Christian brother in his bonds, and it is easy to conceive that, in his circumstances, he would greatly desire to retain him with him.

(a) "in time past" 1Pet 2:10